PC turns itself off after a few seconds

A friend has asked me to look at his computer, and i'm just looking to clarify something.

When I turn the PC on the fans start and everything is going fine for about 3 seconds, it then just turns itself off before the POST.

I've tried:A different PSUThe power switch to ensure its not getting stuckRemoved all things connected to the motherboard (RAM etc)Tried another fan to ensure it's not the motherboard thinking the CPU fan isn't spinning up

The PC just randomly stopped working, the only thing I can think of now is either a motherboard or CPU failure, are there any other tests I can try before going down these more expensive options?

Nope, no beeps, doesn't get that far, its complete silence apart from the fan spinning up.I was heading more towards the motherboard aswell, i'm pretty sure if it was the CPU there would either be nothing happening or it would get to the beeps.

I do have a similiar system (slightly older, working perfectly), which is where I took the power supply/fan from.

EDIT: I removed EVERY single cable except for the 20pin main power cable, it started up and stayed on, i then connected the CPU fan and it stayed on, after placing the 4pin connector it causes the problem, the computer powers off after a couple seconds in case that helps anyone trying to give advice.

Ok, my car won't start. I made sure there's gas in it but when I turn the key, it's sputters for a few seconds, then stalls. What's wrong?

Get my point?

Since you're not being overly generous with the info, I'll ask a bunch of questions:

- what was your friend doing when the system started acting up?- has his system ever been upgraded?- which CPU is being used?- what are the specs of the 250W PSU? FPS makes decent units but 250W seems underpowered. How many amps on the +12v rail?- what are the specs of the replacement PSU?- you said you "tried another fan", how did you go about doing that? Did you remove the entire heatsink/fan & replace it? If you did, did you thoroughly clean everything & apply fresh thermal paste? Or did you leave the HS in place & just change the fan? Or did you leave everything in place, unplug the CPU fan & plug in an extra fan you had laying around? Did both fans have the same number of pins on the plug?- could it be possible that the CPU heat is rising quickly & thermal protection is kicking in?- do you hear a 'click' just before the system shuts down?

Yeah I see your point, at the time it just seemed quite a general question.

Sorry, I didn't realise it had to be that detailed, to answer your questions:- Nothing, was using it like normal, shut it down as usual, next day he tried to start it up and it wouldn't, with the above problem. The computer itself has been working since bought with no problems, well over a year old.- Nope, completely standard, hadn't even opened the case.- I don't know the CPU, I know its an Intel Dual Core, but nothing else, sorry.- The power supply is also just standard, it's been working perfectly fine since bought, and it's 14a going throught the 12v rail.- 500w Jeantech PSU, that's back in other PC and not with me at the minute, so if you need the model number/more details i'll have to get it later.- Left everything in place, unplug the CPU fan and plug the extra fan in. Yes, exactly the same connection.- Possible but unlikely, surely it wouldn't be able to build up enough heat within 3 seconds for the thermal protection to kick in?- Nope, no clicks before the system shuts down, I hear the CD drive making sounds as it gets power, the and hard drive spinning up, but then the power just dies.

I have a Packerd Bell PC from a frend doing the same thing.Model DV100, sn. 814020290330Ran fine for a year. The user complaint that it hung and now it power itself down after 3 seconds.There PC had a lot of dust that I removed, but that did not fix it.I disconnected everything except for video, cpu and memory. Problem still exists.Strange thing is that I find simular posts like this:http://uktsupport.ipbhost.com/index...

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